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Old April 14th, 2018, 09:32 PM   #1
JZerb
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Proper landing of shielded wire within cabinet

so ive been reading up all about proper handling of shielded signal wires, but i still have a few questions. if youre dealing with say a 4-20ma sensor in a machine, using belden 8760 cable for said sensor. that cable runs from the sensor to a junction box on the machine where there is a DIN rail with terminal blocks. once on site there would be wires pulled from the machine junction box to the control cabinet. in the control cabinet that cable would be landed directly on an analog input card.

-That shield for said cable, where would that be landed within the control cabinet? Should that go to a terminal block, the kind thats bonded directly to the DIN rail? Or is there a better method?

-Should the shield on that cable be landed in the machine junction box at all? I assume then it would be carried all the way through to the control cabinet.

lots of people on here with tons more experience then i have on this subject and im looking forward to learning how this is normally handled by the masses out in the real world. thanks!
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Old April 14th, 2018, 11:55 PM   #2
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As a general rule, you should terminate the shield to ground in your PLC cabinet and cut the shield and tape the cable at the instrument end. How best to terminate to the module varies with different PLC brands and different IO modules. It is typically preferable to pre-wire analog input modules to a terminal strip that is designed to provide 24 VDC loop excitation power, if needed, and provide for shield termination along with the analog input plus and minus terminals. Some manufacturers also offer IO module faceplates with pre-wired extension cables to DIN rail mountable terminal boards. It can be difficult to terminate field wires directly to certain analog input modules.
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Old April 14th, 2018, 11:58 PM   #3
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Land the shield at the card on a good ground. Some cards have a terminal for a shield too.
Never land the shield at more than one ground. You can go through a terminal block if needed.

I say at the card because some people have the terminal blocks at the bottom of the cabinet for landing field devices. They land the shield there to ground and use regular unshielded wire to run through the wire duct past a VFD and through a mess of 3phase distribution wires. What noise the shield took care of getting from the field device to the cabinet is picked back up inside the wire duct.

It’s good practice to try to separate the cabinet out for clean and dirty signals as well.
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Old April 15th, 2018, 12:01 AM   #4
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Quote:
Originally Posted by JZerb View Post
so ive been reading up all about proper handling of shielded signal wires, but i still have a few questions. if youre dealing with say a 4-20ma sensor in a machine, using belden 8760 cable for said sensor. that cable runs from the sensor to a junction box on the machine where there is a DIN rail with terminal blocks. once on site there would be wires pulled from the machine junction box to the control cabinet. in the control cabinet that cable would be landed directly on an analog input card.

-That shield for said cable, where would that be landed within the control cabinet? Should that go to a terminal block, the kind thats bonded directly to the DIN rail? Or is there a better method?

-Should the shield on that cable be landed in the machine junction box at all? I assume then it would be carried all the way through to the control cabinet.

lots of people on here with tons more experience then i have on this subject and im looking forward to learning how this is normally handled by the masses out in the real world. thanks!
I didn't mention the JB. Generally speaking, when shielded cables pass through junction boxes, their shields should also be terminated along with the signal wires. The shield should ideally remain continuous from the PLC chassis to the instrument, where it should be insulated.
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Old April 15th, 2018, 12:04 AM   #5
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Quote:
Originally Posted by seth350 View Post
I say at the card because some people have the terminal blocks at the bottom of the cabinet for landing field devices. They land the shield there to ground and use regular unshielded wire to run through the wire duct past a VFD and through a mess of 3phase distribution wires. What noise the shield took care of getting from the field device to the cabinet is picked back up inside the wire duct.
That sounds like a poorly designed control panel.
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Old April 15th, 2018, 12:12 AM   #6
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Quote:
Originally Posted by Bit_Bucket_07 View Post
That sounds like a poorly designed control panel.
It’s what you get when managers want a machine to do something different or add a function. Not to mention it must be complete before the next shift comes in, and anyone can go into a storeroom and check out an enclosure, some terminal blocks, din rail, and a plc. Building panels is something they say they really enjoy doing, except they do it really wrong.
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Old April 15th, 2018, 01:20 AM   #7
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Quote:
Originally Posted by Bit_Bucket_07 View Post
I didn't mention the JB. Generally speaking, when shielded cables pass through junction boxes, their shields should also be terminated along with the signal wires. The shield should ideally remain continuous from the PLC chassis to the instrument, where it should be insulated.
ok, so the drain wire would be landed at a terminal block in the JB normal terminal block, nothings thats bonded to ground via the DIN rail. also, i mainly work with AB I/O modules, which dont seem to have a provision to land the shield. you also mentioned cutting the shield and 'taping' the cable at the instrument end, what exactly does that entail.
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Old April 15th, 2018, 09:33 AM   #8
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http://www.mouser.com/pdfdocs/alphaw...lded-Cable.pdf
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Old April 15th, 2018, 11:03 AM   #9
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Quote:
Originally Posted by JZerb View Post
ok, so the drain wire would be landed at a terminal block in the JB normal terminal block, nothings thats bonded to ground via the DIN rail. also, i mainly work with AB I/O modules, which dont seem to have a provision to land the shield.
The shield wire should only be grounded in the PLC cabinet. If the JB you refer to is pre-wired to the instrument and has provisions for the shield to terminate and pass through the box, then I have to assume that the shield has been cut and insulated at the actual instrument. You shouldn't assume. You should verify.



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Originally Posted by JZerb View Post
you also mentioned cutting the shield and 'taping' the cable at the instrument end, what exactly does that entail.
When terminating the instrument end of a shielded cable, after stripping off the outer jacket, you should cut the foil/braid shield and the shield drain wire off and tape the end of the outer jacket so that the shield can't incidentally make contact with ground. A piece of heat shrink would also work for this purpose, but it's far more common to use electrical tape for field wiring.
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Old April 15th, 2018, 12:11 PM   #10
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Quote:
Originally Posted by Bit_Bucket_07 View Post
The shield wire should only be grounded in the PLC cabinet. If the JB you refer to is pre-wired to the instrument and has provisions for the shield to terminate and pass through the box, then I have to assume that the shield has been cut and insulated at the actual instrument. You shouldn't assume. You should verify.
Excellent. What I currently do is in line with what you have described above aside from a few details, which could be culprits for my noise issues i tend to have. We’ve typically run the analog input module wires within the cabinet to a terminal block, then when the wiring comes in from the field land the signal wires in their appropriate spots and the shield is then attached to a green/yellow terminal block that’s bonded to the din rail. Moving forward I will bring the signal wires from the machine into the PLC cabinet and directly to the input module. I will also be more cautious when cutting the signal wire at the instrument end and making sure to tape the shield to make sure it doesn’t come in contact with anything it shoildnt. The other issue I believe I have is that we use aluminum din rail, and in another thread I found in here in my research on this subject it seems as if that’s not ideal for a proper grounding situation, better to use a steel din rail, grind the paint off the white painted steel backplane in the cabinet and go from there.
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Old April 15th, 2018, 06:25 PM   #11
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The general rule is to keep the shield continuous from instrument to PLC card, and to ground it at one point only. So anywhere there's a terminal strip or a j-box or any other cable joint, the shield needs to be terminated like any other wire (except unlike a regular wire, you might need to sleeve it).

From a functional perspective, where the shield is grounded makes no difference. From a practical perspective, there are some "good practice" and "bad practice" ways of doing it.

The easiest way I find is to earth it at the PLC cabinet terminal strip, using a terminal that earths via the DIN rail. You must still use screened cable from terminal strip to PLC card. So the cable from terminal strip to instrument will have the shield connected to ground at the cabinet end, and cut and taped at the instrument end. Likewise the cable from terminal strip to PLC card will have the shield connected to ground at the terminal strip end, and cut and taped at the PLC card end. You still end up with a shield that is continuous from instrument to PLC module, and grounded at one point only.

On place I often have to deviate from this is with load cells. Many load cells have the cable screen bonded to the body of the load cell, meaning that the cable shield will be grounded at the instrument end by virtue of being bolted to the frame of a machine. This is not ideal because the machine is probably earthed by virtue of being metal and bolted to other things, but it's not deliberately earthed for the purposes of an instrument earth. So you may have to connect an earth wire to the machine for that purpose. Much easier to buy a load cell that doesn't have the shield bonded to the body and to earth it at the panel end, but of course we don't always get to call those shots.
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Old April 16th, 2018, 04:20 AM   #12
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Quote:
Originally Posted by JZerb View Post
We’ve typically run the analog input module wires within the cabinet to a terminal block, then when the wiring comes in from the field land the signal wires in their appropriate spots and the shield is then attached to a green/yellow terminal block that’s bonded to the din rail.
Yeah, it something that many people do, but it is far from good enough

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Originally Posted by JZerb View Post
The other issue I believe I have is that we use aluminum din rail, and in another thread I found in here in my research on this subject it seems as if that’s not ideal for a proper grounding situation, better to use a steel din rail, grind the paint off the white painted steel backplane in the cabinet and go from there.
Even better is to use a dedicated shield clamping bar.

This is what we do:
At the cable entry at the bottom of the cabinet, there is a proper earthing bar with clamps for connecting the screens. The screens are not cut at the earthing bar. Only section of the mantle on the cable is removed to allow for good connection to the clamp. But the screen is allowed to continue with the cable right up to where the wires are connected to the final input device (i.e. the PLC).

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Old April 16th, 2018, 07:20 AM   #13
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Quote:
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Yeah, it something that many people do, but it is far from good enough

Even better is to use a dedicated shield clamping bar.

This is what we do:
At the cable entry at the bottom of the cabinet, there is a proper earthing bar with clamps for connecting the screens. The screens are not cut at the earthing bar. Only section of the mantle on the cable is removed to allow for good connection to the clamp. But the screen is allowed to continue with the cable right up to where the wires are connected to the final input device (i.e. the PLC).

How do you strip the cable like that without damaging shield? Scalpel?
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Old April 16th, 2018, 07:56 AM   #14
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Quote:
Originally Posted by JesperMP View Post
This is what we do:
I like it... and thats what I do also, I learned it from some German cabinets that I worked on that had a bunch of SEW drives, looked great and worked great very little noise issues


Quote:
Originally Posted by karlek
Scalpel?
Thats what I use, I like Olfa and I use blades A1160B but be careful they are sharp, I use a used one with cutting insulation
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Old April 16th, 2018, 08:46 AM   #15
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There are cable tools for exactly this purpose.
But I just use a sharp knife and my surgeon-like senses.
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